1. Field of the Invention
The present invention relates to a ferroelectric thin film element constructed by forming a ferroelectric thin film on a substrate, for example, to a ferroelectric thin film element suitable for use for a nonvolatile memory, a pyroelectric type infrared sensor and the like.
2. Description of the Prior Art
A ferroelectric thin film element using a ferroelectric material of a lead titanate system is applied to a nonvolatile memory, an infrared sensor and the like utilizing pyroelectric characteristics. This type of ferroelectric thin film element has a structure in which a first electrode is formed on a substrate, a ferroelectric thin film is formed thereon by a thin film forming process such as sputtering, and a second electrode is further formed on the ferroelectric thin film.
Meanwhile, in order to sufficiently exhibit the material characteristics of the ferroelectric thin film, it is necessary to make the crystal orientation of the ferroelectric thin film equal to the direction of a spontaneous polarization axis of the thin film. When the crystal orientation of the ferroelectric thin film is made equal to the direction of the polarization axis of the thin film, it is possible to, for example, reduce the volume change of the thin film in the case of applying alternating pulse cycles to the polarization, thereby to make it possible to prevent the fatigue of the material characteristics.
One example of the above described ferroelectric thin film element is disclosed in Japanese Patent Laid-Open Gazette No. 162369/1987. This document discloses a structure in which a MgO thin film oriented in the direction (100) is formed on a semiconductor substrate, an electrode composed of Pt is formed on the MgO thin film, and a PbTiO.sub.3 thin film having a composition of a tetragonal system and having a spontaneous polarization axis in the direction of a c axis, that is, in the direction (001) is oriented in the direction (001) and formed on the electrode composed of Pt. Since the direction of the easy polarization axis of the PbTiO.sub.3 thin film is made equal to the crystal orientation thereof, it is possible to take out sufficient ferroelectric characteristics.
Some of ferroelectric materials such as lead titanate (PbTiO.sub.3 0 and lead titanate zirconate (Pb(Zr.sub.x Ti.sub.1-x)O.sub.3) have a composition having a spontaneous polarization axis in the directions other than above described direction (001), that is, the direction of the c axis. For example, in a composition of a rhombohedral phase of lead titanate zirconate, the direction of the polarization axis thereof is the direction (111). If a ferroelectric thin film element can be produced using such a ferroelectric material having a polarization axis in the direction (111), the selection range of materials used for the ferroelectric thin film element can be enlarged. In addition, in a ferroelectric thin film element for a nonvolatile memory, evaluation whether or not the adhesive properties between a ferroelectric thin film and a material composing an electrode are good affects ferroelectric characteristics caused by applying alternating pulse cycles to the polarization.
When an electrode composed of Pt is formed on a MgO thin film oriented in the direction (100), and a thin film composed of a material having a composition of a rhombohedral phase of lead titanate zirconate is formed thereon, it is impossible to obtain an element exhibiting sufficient ferroelectric characteristics as described in the foregoing prior art document. In addition, the adhesive properties between the ferroelectric thin film and the electrode is poor.